The present invention relates to the field of perforating casings of hydrocarbon wells. In particular, the invention relates to a method and system for oriented perforating of a well casing in the vicinity of a cable such as could be used for permanent sensors.
As part of the completion process for hydrocarbon wells, a steel casing is placed inside the borehole and subsequently cemented in place. The casing, the cement, and portions of the subterranean formation are then perforated using conventional perforation techniques. Ordinarily, the perforations are made in various directions generally perpendicular to the axis of the borehole in the vicinity of the hydrocarbon reservoir using a perforating gun. In some situations, however, the perforations may need to be made so as to avoid a certain direction. For example, if a cable of some kind is located in the borehole but outside the casing during the perforation process, the perforations may need to be directed away from the cable to avoid damage to it. Such a cable could be cemented in place and used for telemetry, or be part of a permanently placed sensor arrangement. If the perforations are made in the direction of such a cable, the cable could be perforated, damaged, or destroyed.
In a deviated hole the perforating guns may be oriented by use of swivels and rollers. However, this technique may not be suitable for avoiding cables and the like since the exact direction of the cable may not be known due to unintended twisting or turning of the cable and casing. Furthermore, the use of swivels and rollers this is not suitable in vertical wells as gravity is not available to assist in the orientation.
Directly sensing the location of the cable has difficulties as well. For example, a cable cemented in place outside the casing is not easily detected with instruments placed inside the casing during perforation, since the detectable signal from the cable is normally either non-existent, or too weak to be a reliable indicator of the cable location. Therefore an alternative method of orientating the perforating gun is needed.
Thus, it is an object of the present invention to provide an apparatus and method for orienting a perforation gun in a borehole so as to avoid perforating a cable or sensors located in the area to be perforated.
According to the invention, a method is provided for perforating a conduit in a hydrocarbon well so as to avoid perforating in a direction where a cable resides. A mass of preferably metallic material is fixed to the conduit at a predetermined offset angle with respect to the cable. The angular position of the mass of material is detected using a detector adapted to sense the mass of material. A perforation device is oriented based on the angular position of the mass of material and the predetermined offset angle, such that the perforation device will create perforations in the conduit in a direction substantially away from the cable. Finally, the conduit is perforated so as to avoid substantial damage to the cable.
According to a preferred embodiment, the method is used to perforate casings used in hydrocarbon wells. The detector is located inside the casing, and the mass of metallic material (preferably a form of steel) is located outside the casing. Additionally, the cable comprises a number of electrical conductors and is used to communicate with a number of permanent sensors located on the cable in the vicinity of the casing to be perforated. The cable and metallic mass are clamped to the exterior surface of the casing using specially adapted clamps. The predetermined offset angle is preferably approximately 180 degrees.